The conjuration skill increases anytime a summoned creature or item is used in combat. This applies even if the spell was cast prior to the start of combat. Summoning outside of combat does not increase skill.

Conjuration can be leveled quickly by casting the Soul Trapspell on dead bodies (corpses are considered valid targets for Soul Trap but always resist the spell). The body can be dragged (hold the action button) to a safe area first so the player may use 'wait' to recover magicka. Soul Trap can also be cast on animals.

Conjuration can also be leveled by casting any summon spell repeatedly in combat, preferably from a position where enemies cannot reach the player.

As with any skill grinding, take advantage of the standing stones west of Riverwood (in this case, the Mage) and, if possible, the Well Rested bonus from sleeping for maximum efficiency.